China has extremely strict laws on smuggling. Often there will be someone at the conveyor belt asking you for your luggage tag before you pick it up. I've seen that only in high risk airports in certain situations (at certain times better said), and not even in Frankfurt about a week from Bataclan massacre while they were still hunting for the perpetrators. Point is China takes smuggling seriously and even admitting that they have 1% control over NK smuggling would be highly negative PR for them.
Brigs, don't keep your hopes up for a market rebuild of the country. If regime crashes down, there will be NK internal strife/civil war first. A lot of people are working for Kim dynasty there, millions, and other millions don't like them, or won't like them when they get a grasp of the whole truth. I'm not saying NK is kidnapping 13yo Japanese girls from beaches, cause that's bullshit, but there must be a ton of atrocities Kims did that aren't well known. It's always like that, totalitarian system crumbles, dirty laundry comes out. Even if NK society gets through that, SK and China would be taking millions of refugees who are, at this moment, in NK in immediate danger of hunger or anything else.
20 million "underdeveloped" people in Asia is not exactly a market. Sorry for that word, I don't have any better, you know what I mean. Even if they all were friendly to eachother, peaceful and willing to participate in democracy and market, it's not exactly a big thing, is it? I've been in cities in Asia that have more people than North Korea. Nope, rebuild will have to go on SK's budget, like it went on West Germany. SK will have to invest in reunification, and they're willing to do just like Germans did, when Kim falls.
IMHO, U.S. should publicly complete ignore them, put pressure behind the scenes on China to minimize NK profits done via that channel, and we'll have to wait a decade or two for the thing to crumble down just like Soviet Communism did. Ban your citizens from visiting NK and if they really steal a citizen or allied citizen, extract them ffs.